This invention relates to an improved flotation method and apparatus and has been devised particularly for improving the purity or grade of concentrate produced from froth emanating from a flotation cell.
It is well known to separate various types of minerals by the process known as flotation using a flotation cell. The mineral to be treated by flotation is finely ground and prepared in a slurry with water. Various reagents are then added to assist in the flotation of the desired species from the slurry. The slurry then passes to a bank of one or more flotation cells.
The flotation cells which are predominantly used in commercial plants are of the mechanical aeration type in which gas bubbles and particles are brought together by vigorous agitation in a stirred tank. Air is introduced to the region of the impeller through the hollow shaft tank. The particles to be floated attach to the bubbles and rise to the surface where they form a separate froth layer. The froth, bearing the valued minerals, is removed from the cell separately from the pulp or slurry containing the unwanted particles.
Other types of flotation cell are used, in which the gas is introduced through fine holes in a pipe, or through a porous medium, in the bottom of the cell. Other variations are to inject the gas into the cell in the form of a mixture with a flowing stream of the slurry, or in solution in the slurry.
In conventional known flotation cells having substantially vertical side walls, the froth from a particular mineral/liquid mixture (known as pulp) in an operating cell will reach a certain height on top of the pulp when aerated according to the cell configuration, construction and method of operation. This height of the froth on top of the pulp is hereby defined as the "natural froth height" as referred to in the remainder of this specification. The major volume of the cell is generally located above the source of bubbles which is frequently a rotating impeller. Most cells are parallel sided in this region although an angled baffle may be provided to "crowd" the bubbles toward a weir located on one side of the cell. Throughout this specification, where reference is made to the horizontal cross-sectional area of the body of the cell, the area referred to is the major or larger area before any reduction by angled baffles etc.
A problem which is encountered with all these known types of cell, relates to the entrainment of unwanted slurry particles into the froth. Where the froth forms just above the surface of the liquid slurry, the rising bubbles carry with them particles of the material to be removed, attached directly to the surface of a bubble and forming a line of contact where the gas in the bubble, the liquid in which the solid particles are suspended, and the surface of a solid particle are all co-existent. In addition, however, some of the slurry is carried into the froth layer in the form of thin films between the individual bubbles. Since this liquid contains unwanted solids at approximately the same average concentration as in the liquid in the cell itself, it is inevitable that unwanted gangue material is entrained into the froth with the particles of values which it was intended to float.
As a consequence of the entrainment of the undesirable gangue particles, the grade or purity of the flotation product or concentrate is reduced. In some cases the purity can be improved by subjecting the froth concentrate to successive flotation treatments, which adds to the cost and complexity of the plant, and may lead to losses of values from the re-treatment flotation cells.